1. Field of the Invention
The present invention relates to a method of forming a capacitor of a semiconductor device, and more particularly to a method of forming a capacitor of a semiconductor device, which can secure a leakage current characteristic while securing a desired charging capacitance.
2. Description of the Prior Art
Recently, as the high integration of a memory product is accelerated due to the development of semiconductor fabrication technique, a unit cell area is greatly reduced and an operation voltage is lowered. However, despite the reduction of cell area, it is still requested that the charging capacitance required for operating a memory device be sufficiently high and not less than 25 fF/cell to be sufficient for preventing the occurrence of a soft error and the reduction of refresh time.
Accordingly, even though three-dimensional storage electrodes each having an electrode surface of a hemisphere shape have been applied to nitrogen-oxide (NO) capacitors for dynamic random access memories (DRAM's), which employ a Si3N4 film currently deposited as a dielectric using di-chloro-silane (DCS), the heights of the NO capacitors are continuously increased so as to secure a sufficient capacitance.
As is well known, the charging capacitance of a capacitance is proportional to a surface area of an electrode and a dielectric constant of a dielectric material and reversely proportional to a space between the electrodes, i.e., the thickness of the dielectric.
Meanwhile, the NO capacitors reveal limitations in securing a charging capacitance required for a next generation DRAM of no less than 256 Mbit; accordingly, in order to secure a sufficient charging capacitance, the development of a capacitor employing a dielectric film such as Al2O3 or HFO2 as a dielectric material is vigorously progressed.
However, because an Al2O3 dielectric film has a limit in securing a desired charging capacitance because its dielectric constant (ε=9) is merely two times of that of SiO2 (ε≈4) and is not so high. Therefore, such an Al2O3 dielectric film is restrictively applied as a dielectric film of a capacitor in a memory to which a process of metallic wiring with a line width of 100 nm or less is applied.
In addition, although the HfO2 dielectric film has a dielectric constant of about 20 and is more advantageous than the Al2O3 dielectric film from a standpoint of securing a charging capacitance, the HFO2 dielectric film has a problem in that because its crystallization temperature is lower than that of the Al2O3 dielectric film, leakage current is abruptly increased when a subsequent high temperature thermal process of 600° C. or more is performed. As a result, it is the current situation that the HfO2 dielectric film is not so easily applied to a memory product.
For this reason, recently, an HfO2/Al2O3 capacitor of dual dielectric film structure, an HfO2/Al2O3/HfO2 capacitor of triple dielectric film structure, and etc. have been developed, in which those capacitors are formed by laminating a layer of Al2O3 film generating a very low leakage current level and one or two layers of HfO2 film having a higher dielectric constant as compared to the HfO2 film.
However, in connection with the fact that the crystallization temperature is lower than that of the Al2O3, if a high temperature thermal process of 750° C. or more is performed when a top electrode is formed from doped polysilicon, or if a high temperature thermal process of 600° C. or more is performed when the top electrode is formed from a metallic material such as TiN, a problem still arises in that the HfO2 dielectric film is crystallized and impurity is diffused into the dielectric film from the top electrode, thereby increasing the leakage current. Here, in the case in which the top electrode is formed from doped polysilicon, the impurity is Si or dopants, while in the case in which the upper is formed from TiN induced by TiCl4, the impurity is Cl ions.
Consequently, at present, each of the Al2O3 and HFO2 films is substantially difficult to employ as a dielectric film capable of securing a desired leakage current characteristic while securing a desired charging capacitance.